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GASOLINE ENGINES 


PREPARED IN THE OFFICE OF 
THE DIRECTOR OF AIR SERVICE 


August, 1919 



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WASHINGTON 

GOVERNMENT PRINTING OFFICE 
1919 


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for Aeronautics 

Washington, D. a 


930 








W A It I >E!*ARTAI ENT 

Document No. 939 
Office of The Adjutant General 


By Exchange 

Bureau of GonstructloQ £, Repair 
March 2 5 1935 







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WAR DEPARTMENT, 

Washington, August i , 1919. 

The following publication, entitled “Gasoline Engines,” is pub¬ 
lished for the information and guidance of all concerned. 

[062.11, A. g. o.] 

By order of the Secretary, of War : 


Official : 

P. C. HARRIS, 

The Adjutant General. 


PEYTON C. MARCH, 
General , Chief of Staff. 


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CONTENTS. 


Page. 

Chapter I.—General principles of the internal-combustion engine_ 7 

Introductory classification_ 7 

Fuel_ 7 

Combustion_ 8 

The relation of pressure, volume, and temperature inside the cylinder. 9 ’ 

Mean effective pressure_ 9 

Atmospheric pressure_ 10 

Temperature_1__ 10 

Volume_ 10 

Relation of volume to pressure at constant temperature_ 10 

Relations between pressure, volume, and temperature_ 11 

Heat_ 11 

Chapter II.—The cycle of operations_ 14 

The length of an engine cycle___ 14 

The two-stroke cycle engine_ 14 

Operation of the two-stroke cycle type engine_ 15 

The four-stroke cycle type_ 15 

The four strokes and the duties of each_ 15 

Intake stroke_ 16 

Closing of intake valve_ 16 

Compression stroke_ 16 

Why the charge is compressed before ignition- 16 

Igniting the charge_ 17 

Power stroke_ 17 

Exhaust stroke- IS 

Two-cycle advantages_ 18 

Two-cycle disadvantages- IS 

Four-cycle advantages_ IS 

Four-cycle disadvantages- 18 

Measuring parts of revolutions—degree measure- 19 

More exact information on the strokes of a cycle- 19 

Comparison of air and water cooled types- 20 

Chapter III.—The varying lengths of cycle strokes- 22 

Peculiar relation between piston and crank-pin travel- 22 

Early and late valve openings- 23 

Why the exhaust valve opens early. (Before bottom center.)- 23 

The exhaust stroke_ 25 

Why the exhaust valve closes after top center- 25 

The opening of the inlet valve- 26 

Why the inlet valve closes- 27 


5 










































I 

6 CONTENTS. 

’* Page. 

Chapter IV.—Types of motors_ 29 

Classification_ 29 

Radial engines_ 29 

Rotary engines_ 29 

Disadvantages_ 29 

Vee type_ 30 

Vertical type_ 30 

Rotary and stationary engines_ 30 

Types of cylinders_ 31 

Chapter V.—Valves and valve-operating machinery_ 32 

The poppet valve_ 32 

Material and the making of poppet valves_ 33 

Number and mounting of valves_ 34 

Misfit valves and faults of seating_ 34 

Grinding valves_ 36 

The valve-operating mechanism_ 39 

Air leakage around valve stems_ 39 

Valve springs_ 40 

Case-hardening valve-stem ends_ 41 

Cams._,___ 41 























GASOLINE ENGINES. 


CHAPTER I. 

GENERAL PRINCIPLES OF THE INTERNAL-COMBUSTION 

ENGINE. 

INTRODUCTORY CLASSIFICATION. 

Virtually all modern engines are some form of heat engines, i. e., 
fuel is burned to form heat from which, indirectly, is obtained 
mechanical motion or power. The so-called “ steam ” engine is a heat- 
engine ; the engine we commonly call a “ gasoline ” engine is a heat 
engine. But, the steam engine and the gasoline engine are two dif¬ 
ferent forms of heat engines. In the first case the fuel is burned ex¬ 
ternal to the working parts of the engine proper. Therefore, the 
steam engine is termed or classified as an “ external-combustion en¬ 
gine.” On the other hand the gasoline engine burns its fuel within 
the engine and is classified as an “ internal-combustion engine,” or 
gasoline engine. It has been called a gasoline engine because the fuel 
generally used is gasoline. 

From the point of efficiency based upon the percentage of heat in 
fuel that is converted into mechanical energy, the steam engine shows 
an efficiency of from 5 per cent to 10 per cent, and the internal-com¬ 
bustion engine (gas engine) shows an efficiency of from 17 to 30 
per cent. 

Internal-combustion engines are best for airplanes and all types 
of aircraft, as they are the lightest and most efficient of prime 
movers. The fuel consumption is twice as great in a steam-power 
plant (owing to heat losses) as would be the case in a gasoline engine 
of equal power and much less weight. 


FUEL. 

Illuminating gas, kerosene, crude oil, alcohol, gasoline, and many 
other combustible materials have been used for operating internal- 
combustion engines. Oxygen, which is contained in the air, has in all 
cases been used as the supporter of combustion. 

Gasoline is the fuel used in all airplane internal-combustion en¬ 
gines. It is most desirable for the following reasons: 



8 


GASOLINE ENGINES. 


(a) Can be obtained in large quantities. 

(b) Cost is comparatively low. 

( c ) Can be easily vaporized at atmospheric temperature. 

( d ) Explodes rapidly, allowing the engine to run at high speed, 
thus permitting the use of a small, light engine of ample power. 

( e ) Is in a form which is easily handled, and for a given volume 
and weight will furnish sufficient heat units to run a powerful motor 
for a considerable period. 


COMBUSTION. 

When two materials, one a combustible material and the other a 
supporter of combustion, are mixed together and then raised to the 
temperature at which they were combined a chemical change takes 
place and heat is liberated. A certain amount of heat is necessary 
to start the chemical action, but once started the heat liberated will 
be sufficient to continue the action until one or both of the materials 
have been exhausted. Such a chemical change is called combustion. 
Combustion is very common in nature. A stick of wood when once 
lighted will liberate heat. A piece of iron when exposed to the air 
will combine with oxygen in the air and form rust or iron oxide. 
Vegetable matter combines with oxygen and decays. These are all 
forms of combustion, but many of them occur so slowly that we do 
not think of them as such. When combustion occurs very rapidly 
we call the action which takes place “ explosion,” and it is this form 
of combustion with which we are chiefly concerned in our study of 
gasoline engines. Heat which is liberated during explosion may do 
two things: First, the chemical change may be such that a dense 
substance is changed to a light substance, which, under the same 
pressure, will occupy a greater volume (for example, gunpowder 
changes to a gas which is chiefly dioxygen; gasoline changes to 
carbon dioxide gas). Second, the heat of the explosion will cause 
the gas to expand if under a given pressure or if confined the pres¬ 
sure will rise. When properly confined a compressed gas can be 
made to perform useful work. 

When a fuel such as gas is admitted to the cylinder of an engine 
a certain quantity of air is admitted at the same time to furnish 
the necessary oxygen for combustion. Before ignition this mixture 
is compressed into a small space. This compression serves to mix 
the particles of air and fuel more intimately and to raise the tem¬ 
perature of the mixture. This compressed mixture will ignite with 
more certainty and will burn more evenly than a rarer and colder 
mixture. 

The heat energy thus liberated does mechanical work in the fol¬ 
lowing way: Combustion causes the temperature of a certain quan- 


GASOLINE ENGINES. 


9 


tity of gas trapped in the cylinder of the engine to rise. This rise 
in temperature is accompanied by a rise in pressure of the gas. The 
action which takes place and which appears to be an explosion is in 
reality a progressive combustion and subsequent expansion of the 
products of combustion. The pressure thus created forces the piston 
toward the crank end of the cylinder, thus performing work. Mo¬ 
tion of the piston by means of the connecting rod is caused to turn 
the crank. Forceful rotation of the crank performs work which is 
the function of a gasoline engine. 

THE RELATION OF PRESSURE, VOLUME, AND TEMPERATURE INSIDE THE 

CYLINDER. 

The actual laws by which the pressure, volume, and temperature 
of gas vary and the relation between them is a rather difficult study 
for the average mechanic. Without giving mathematical relations 
the following general principles should be thoroughly understood by 
the student: 

The charge taken into the combustion chamber of an internal- 
combustion engine is not a perfect gas, but for our present purpose 
we may think of it as such. A body of gas possesses weight, which, 
for a given amount of gas, remains always the same. However, the 
gas may occupy a large or small space and may be at high or low 
temperature. If contained in an inclosed space, a pressure will bo 
exerted by the gas on the walls of the container, which is equal in 
all directions. 

MEAN EFFECTIVE PRESSURE. 

In many instances the “brake mean effective pressure” of an 
engine is spoken of. By this is meant the mean effective pressure 
which, if exerted on each piston during its power stroke for the entire 
length of the stroke, wrnuld develop the brake horsepower of the 
engine. Thus, with a constant pressure, there is a certain force 
acting through a certain distance a certain number of times every 
minute, and the horsepower developed can be determined by dividing 
the foot-pounds of work done per minute by 33,000. 

“ p ” equals the brake mean effective pressure in pounds per square 
inch. 

“ 1 ” equals the length of stroke in feet. 

“ a ” equals the area of the piston in square inches. 

“ n ” equals the number of explosions per minute, which is equal 
to the number of cylinders divided by 2 and multiplied by 
the revolutions per minute. 

Then brake horsepower equals “ plan ” divided by 33,000. Know¬ 
ing the brake horsepower, bore and stroke, and the revolutions per 
minute, the brake mean effective pressure can readily be determined. 

131662—19-2 


10 


GASOLINE ENGINES. 


The fuel consumption of an engine is usually stated in pounds of 
fuel per brake horsepower hour, meaning that a certain amount of fuel 
is used when the engine develops the brake horsepower for the period 
of an hour. This value is obtained by running fuel consumption 
tests and dividing the fuel consumed per hour by the brake horse¬ 
power developed. 

ATMOSPHERIC PRESSURE. 

The atmosphere exerts a pressure on objects due to the weight of 
the air above us. At sea level the weight of the air is equal to 14.7 
pounds for each square inch of the earth’s surface. Consequently 
when we speak of a vacuum or suction, we simply mean that the 
pressure which the gas exerts on the wall of the container is less than 
atmospheric pressure. 

TEMPERATURE. 

When heat is applied to a mass its temperature rises, and when 
lieat is taken away the temperature of the mass goes down. Heat 
always travels from points of high temperature to points of low 
temperature Avhen allowed to take its own course. In measuring 
temperature we use the term “ degree,’’ which is simply an arbitrary 
unit. There are two scales for measuring temperature—the Centi¬ 
grade and the Fahrenheit scale. Zero degrees on the Centigrade 
scale corresponds to 32 degrees on the Fahrenheit scale, which is the 
temperature at which water freezes at sea level. It must not be 
assumed, however, that these points represent the temperature at 
which all heat is absent, for such a point is far below 7 zero on either 
the Centigrade or the Fahrenheit scales and is known as the “ abso¬ 
lute zero.” One degree Centigrade is equivalent to 1.8 Fahrenheit. 
As the Fahrenheit scale is the one with which we are all familiar, 
it will be used in this text. Absolute zero, or the point at which there 
is no heat, is 460 degrees beloAv zero on the Fahrenheit scale, so that 
32 degrees Fahrenheit is equal to 400 degrees plus 32 degrees, or 
492 degrees absolute. 


VOLUME. 

Volume is the space occupied by a mass and is measured in cubic 
inches, cubic feet, etc. 

RELATION OF VOLUME TO PRESSURE AT CONSTANT TEMPERATURE. 

In thinking of the changes in pressure and temperature we must 
think of the change in relation to absolute zero, pressure, and tem¬ 
perature. When the volume, pressure, qr temperature of a given 
w r eight of gas is changed one or both of the other two must also be 


GASOLINE ENGINES. 


11 


changed. If pressure changes and temperature remains constant, 
then with increased pressure volume must decrease and with de¬ 
creased pressure volume must increase. 

RELATIONS BETWEEN PRESSURE, VOLUME, AND TEMPERATURE. 

If volume is held constant and temperature increased, the pressure 
will rise, but if temperature is lowered the pressure will fall. If 
pressure is held constant and volume increased, the temperature will 
fall, but if volume is decreased the temperature will increase. The 
above may be expressed as follows: For a given weight of gas, the 
pressure multiplied by the volume and divided by temperature re¬ 
mains always the same, no matter what changes take place in the gas, 
providing that pressure and temperature refer to absolute values and 
not values measured from atmospheric pressure and zero Fahrenheit. 
The application of the above principles is presented in the paragraph 
on the four strokes of the cycle. 

HEAT. 

Because heat controls the basic principle of the gasoline engine, 
it is best in a brief way to consider its application. Whenever a 
solid substance is heated, considering everyday substances, a cubical 
expansion takes place. The substance will grow longer, wider, and 
thicker with each degree of heat added, so that in some cases the 
expansion is very noticeable. Of course, if heat is continually added 
the substance will in time fuse and lose one of its physical char¬ 
acteristics, commonly called “ shape.” Air, which is a gaseous sub¬ 
stance, when heated expands on the same general principle as a solid 
substance. Each degree of heat added to a volume of air or similar 
gas expands it, the volume becoming greater if the pressure is kept 
uniform. 

Most gases expand very rapidly when heated, so much so that if a 
material change of temperature takes place a considerable expansion 
would be noted. Nearly everyone has witnessed the rising of a hot¬ 
air balloon. Such a balloon is generally inflated by supporting its 
envelope over a bonfire. The air in the balloon is hot air or expanded 
air and occupies more space than cold air would, and is therefore 
lighter than cold air. As a result the balloon will float upward in the 
atmosphere, and if well inflated with hot air will very naturally 
support some weight. 

If a volume of gaslike air be heated within an inclosed vessel so 
arranged that the gas is not free to expand, then the pressure within 
the vessel will be increased. If the heat is continued to any high 
temperature and the vessel be not very strong it will burst by reason 
of the pressure generated. Suppose we have a cylinder and piston 


12 


GASOLINE ENGINES. 


arrangement so that it can not be moved; imagine there is absolutely 
no leakage. Now, if this charge is ignited the temperature and 
pressure will run up very high. After five minutes we would find 
very probably only a few ounces of pressure remaining. The reason 
for this is that the cool cylinder and piston have absorbed the heat of 
the explosion, and as the heat subsides the pressure subsides. In 
other words, the hot gases have contracted in cooling. Heat and 
pressure can be considered practically the same thing in studying 
the gasoline engine. 

The two features which stand prominently in the adaptability of 
an engine for airplane use are the weight per horsepower and the 
weight of fuel per horsepower developed. 

All the heat which shows up in the engine in one form or another 
comes from the fuel used. Part of the heat of the fuel is trans¬ 
formed to work at the engine shaft, and the ratio between this heat 
and the heat in the fuel is called the “ thermal efficiency.” During 
the exhaust stroke the gases are rejected at a high temperature and 
the heat in these gases is wasted or lost. Some heat is also dissipated 
by radiation, since the temperature of the engine is higher than the 
surrounding medium. During combustion, high temperatures are 
reached and certain parts of an engine require cooling, or hot points 
would develop and cause preignition, or perhaps the destruction of 
the oil filament. The internal-combustion engine being a mechani¬ 
cal device, loses a certain amount of heat or work through friction. 
The ratio between the work done at the shaft to that on the piston 
is called “mechanical efficiency.” Summing up the various heat 
losses is usually referred to as a “ heat balance.” The following is a 
typical heat balance: 

Per cent. 


Heat transformed to work_ 21 

Heat lost to exhaust and radiation- 37 

Heat lost to cooling system_ 36 

Heat lost to friction_ 6 

Heat in the fuel_100 


The thermal efficiency is twenty-one one-hundredths, or 21 per 
cent. Therefore, with the best gasoline motors we make we use only 
about 21 per cent of the heat value of the fuel, or, in other words, we 
lose four-fifths of the heat or power that is in the fuel. These figures 
are only approximate, but should serve to give an idea of where all 
the power of heat goes. 

Work is done whenever a force moves through a distance. 

Power is the rate of doing work. 

Energy is the capacity to work. 

Efficiency is the ratio of useful work to input. 







GASOLINE ENGINES. 


13 


In the first gas engines made, gas was drawn into the cylinder and 
burned without being compressed. The result was that it required 
a great deal of gas to develop a given amount of horsepower. In 
other words, the engines were not economical. It was found that by 
compressing the gas before the charge was fired it was possible to ob¬ 
tain a great deal more power with the same amount of fuel. When 
gas is compressed it will burn much more rapidly. Naturally, in a 
motor running at the high speed that our airplane motors do, the gas 
must be burned completely in a very short space of time in order to 
do any work. Therefore, it is made plain that if a motor is to run it 
will be necessary first to draw gas into the cylinder, then compress it, 
then it must be burned and expanded and the burned gas cleared 
out of the cylinder. These four things must be done in any gaso¬ 
line motor. After the gas is compressed in the cylinder its tempera¬ 
ture rises to a certain extent. Then the heat of the electric spark 
which is used for ignition causes a further rise. The heat of the 
spark, together with the heat of the compression, is sufficient to ignite 
the gas or start its burning. Then, as the gas burns its temperature 
and pressure will rise very rapidly. If the gas were not confined, 
the temperature would go quite high, but when burned in a confined 
space pressures are increased as the gas burns. This increase in pres¬ 
sure adds to the head, as also does the burning of the fuel. Therefore 
we obtain a temperature in a gas-engine cylinder of between 2,600° 
and 3,000° F. You can think of this as a white-hot flame. 


CHAPTER II. 


THE CYCLE OF OPERATIONS. 

A cycle is a series of events which upon completion are repeated 
in the same order. 

An internal-combustion engine cycle consists of (1) intake, suction, 
or admission of the charge; (2) compression; (3) ignition, combus¬ 
tion, and expansion; (4) exhaust. 

The number of strokes necessary to complete this cycle gives a 
means of cyclic classification, as follows: (1) Two-stroke cycle; (2) 
four-stroke cycle. The latter is sometimes called the “ Otto cycle.” 
The two-cycle is sometimes called the “ Clark cycle.” 

THE LENGTH OF AN ENGINE CYCLE. 

The observed performance of an engine is always based upon the 
action of its piston. When a piston moves from one extreme end 
of its travel to the other, as from the highest point to the lowest 
point, the movement is termed a “ piston stroke.” One complete 
revolution of the crank shaft would carry the piston through two 
piston strokes, one “ down stroke ” and one “ up stroke.” The two- 
cycle engine completes its program of events in two strokes of the 
piston and one revolution of the crank shaft. The four-cycle engine 
completes its program in four piston strokes or two complete revolu¬ 
tions of the crank shaft. 

THE TWO-STROKE CYCLE ENGINE. 

The two-cycle engine is the simplest, because there are no valves 
in connection with the cylinder, as the gas is introduced into that 
member and expelled from it through parts cored into the cylinder 
walls. These are covered by the piston at a certain portion of its 
travel and uncovered at other parts of its stroke. In the four-cycle 
engine the gas is admitted through a port at the head and closed by 
a valve, while the exhaust gas is expelled through another port con¬ 
trolled in a similar manner. These valves are operated by mechanism 
distinct from the pistoiv 
14 


GASOLINE ENGINES. 


15 


OPERATION OF THE TWO-STROKE CYCLE TYPE ENGINE. 

{a) During the upward motion of the piston a fresh charge has 
been compressed. Just before head dead center is reached, ignition 
takes place. 

(b) Inertia carries the piston over head dead center and the ex¬ 
panding gas drives the piston downward on its power stroke. Some 
time before crank dead center is reached the piston uncovers the end 
of the transfer passage. Now, during its downward motion the piston 
has compressed to a pressure of about T or 8 pounds a mixture of line 
vapor and air in the crank case. When the transfer passage is opened 
this pressure causes a new charge to rush into the cylinder. Arriving 
there it expands, strikes the deflector, is deflected upward, thus aiding 
in exhausting the burnt gas from the cylinder. 

(<?) Soon after the piston has passed crank dead center, its up¬ 
ward motion covers the transfer passage, and not long after that 
covers the exhaust port, thus trapping in the cylinder a quantity of 
explosive mixture. The continued motion of the piston compresses 
this charge at the same time, creating a suction in the crank case 
which draws into the crank case the next fresh charge. At the proper 
time ignition takes place as above described, and the cycle is re¬ 
peated. All these things have been accomplished in two strokes 
of the piston or one revolution of the crank shaft. 

FOUR-STROKE CYCLE TYPE. 

Practically all engines now in use in airplanes are of four-cycle 
or, more correctly speaking, four-stroke cycle type. 

On the first down stroke of the piston, called the “ intake stroke,” 
the inlet valve opens and a charge of fuel and air is sucked into the 
cylinder. The inlet valve is then closed, and the piston moves up 
on the second stroke, compressing the gas. Next the spark occurs, 
igniting the charge, causing the pressure to raise, which forces the 
piston out on its third stroke, which is called the “ expansion ” or 
u power stroke,” pushing the burnt gas out of the cylinder through 
the exhaust valve. The exhaust valve then closes, completing tho 
cycle of four strokes. 

To sum up: The four-cycle motor requires four strokes to complete 
its program or cycle. These strokes are (1) intake or suction, (2) 
compression, (3) expansion or working stroke, (4) scavenging or 
exhaust stroke. 

THE FOUR STROKES AND THE DUTIES OF EACH. 

As explained before, in each cylinder four strokes of tho piston 
are required to complete the cycle of operation. 


16 


GASOLINE ENGINES. 


INTAKE STROKE. 

We will start with the piston on top dead center at the beginning 
of the first down stroke of the cycle. After moving down a very 
short distance the exhaust valve closes and the intake valve opens, 
connecting the cylinder to the intake manifold, which is in turn 
connected to the carburetor. As the piston continues downward, the 
volume of the cylinder is increased, causing the pressure to fall below 
that of the atmosphere. This causes a flow of gasoline and air into 
the mixing chamber of the carburetor where they are mixed, forming 
a vapor which flows through the intake manifold and intake valve 
port to the cylinder. The pressure in the cylinder remains below 
atmospheric pressure to the end of the stroke and for a short time 
after the piston has started up on the following stroke. The inertia 
of the incoming gas mixture also tends to keep it flowing into the 
cylinder, even though the mixture has reached a pressure equal to 
that of the atmosphere. 

CLOSING OF INTAKE VALVE. 

For the above reason, the intake valve is not closed until the 
piston has traveled upward a short distance on the second stroke, 
allowing the maximum charge to enter the cylinder. 

COMPRESSION STROKE. 

As the piston moves upward on the second stroke, the intake valve 
closes. Then, due to the decrease in volume as the piston continues 
upward, and the increase in temperature, due to compression, and ab¬ 
sorption of heat from the cylinder walls, the pressure of the gas 
is increased. The pressure at the end of the compression stroke in 
an engine with five to one compression ratio will be about TO pounds 
per square inch, gauge pressure of about 85 pounds per square inch 
absolute when the engine is developng its maximum power. 

WHY THE CHARGE IS COMPRESSED BEFORE IGNITION. 

By compressing the gas before ignition, the pressure after ignition 
is greater in proportion to the compression ratio, so that a motor 
developing a given power may be made very small as compared to 
one which does not compress the gas before ignition. The tem¬ 
perature of the gas when ignited rises to about 2,700° F., and 
if the cylinders were not artificially cooled they would melt 
after a few explosions. In cooling the cylinders, heat is taken 
from the exploded gas, causing loss of power. The greater the 
surface of cylinder wall exposed to the cooling water, the greater 


GASOLINE ENGINES. 


37 


will be the loss of power. By compressing the gas before ignition, 
the area of the cylinder walls is greatly reduced, consequently thn 
engine will be much more efficient. The object, then, in compressing 
the gas before ignition is to reduce the size of the engine and in¬ 
crease the efficiency. When gas is compressed above a certain limit, 
the temperature of compression becomes so great- as to cause preig¬ 
nition. 

The average practice of the compression in any automobile motor 
cylinder is 65 pounds to the square inch. The aviation motor oper¬ 
ates with much higher compression, for the reason that they ascend 
to high altitudes. The effect of ascending to an altitude of less dense 
air is to lessen the compression within the cylinder. An airplane 
motor, built for an ascent of 20,000 feet, must have a much higher com¬ 
pression than one only intended for an ascent of 5,000 feet. Motor's 
intended for high altitude flights are labeled “high compression.” 
This compression may reach 135 pounds to the square inch. Those 
intended for low altitude flights, such as Navy observation work, 
are marked “ low compression.” This compression will range from 
85 to 105 pounds to the square inch. 

IGNITING THE CHARGE. 

Approximately, at the end of the compression stroke, the charge 
is ignited by means of an electric spark. A short time is required 
after the spark occurs for the flame to be propagated throughout the 
charge, and for this reason, except for starting, the spark occurs 
a little before dead center, the amount of advance depending upon 
the engine speed. When the charge is fired, the temperature sud¬ 
denly increases to about 2,600° to 2,800° F.. causing a great 
increase in pressure. Thus the chemical energy contained in the 
charge is converted to heat energy. An engine with five to one comr 
pression ratio develops a maximum pressure of about 250 to 300 
pounds per square inch. 

POWER STROKE. 

After firing the charge, the high pressure developed forces the 
piston down, thus converting heat energy into mechanical energy. 
As the piston travels downward, the volume of the cylinder increases, 
causing a drop in pressure. The temperature also drops, due to the 
expansion and to loss of heat through the cylinder walls. If the ex¬ 
haust valve were not opened until the end of the stroke a greater 
average force would be exerted by the gas on the piston during the 
power stroke, but the pressure in the cylinder at the beginning of the 
following upward stroke w T ould act against the piston, resulting in 

131662—19-3 


18 


GASOLINE ENGINES. 


negative work. It has been found by experiment that the engine will 
deliver maximum power if the exhaust valve be opened from about 
40 to 60 degrees before bottom dead center, at which point the pres¬ 
sure in the cylinder will equal about 40 to 50 pounds per square inch 
(5 to 1 compression ratio). 

EXHAUST STROKE. 

As the piston starts up on the fourth stroke, the exhaust valve is 
opened and the burnt gases are forced out through the exhaust 
port, exhaust manifold and muffler. A great amount of heat energy 
is contained in the exhaust gas, which, however, can not be converted 
in the engine. In order that the fresh charge taken in on the follow¬ 
ing intake stroke may be of maximum value, it is desirable to ex¬ 
haust as much of the burnt gas as possible. At the end of the stroke, 
the burnt gas in the combustion chamber is still under pressure 
greater than the atmosphere. For this reason and because of the 
inertia of the outgoing gas, the exhaust valve is not closed until a 
few degrees after the piston has passed top center. The above events 
are then repeated over and over again, each repetition being one cycle. 

TWO-CYCLE ADVANTAGES. 

No valves, valve gear, cams and cam shaft, more uniform turning 
movement, smaller cylinder volume per unit of power. Closing of 
valves is not dependent on springs; therefore, engine may be made 
to run at higher speeds (5,500 revolutions per minute). 

TWO-CYCLE DISADVANTAGES. 

Loss of fresh fuel with exhaust reduces the economy; crank case 
must be kept gas tight to prevent loss of fuel and compression; 
fresh fuel entering the cylinder full of hot exhaust gases may cause 
premature explosion, and if this occurs before the admission port is 
closed, the crank case charge may explode, causing considerable dam¬ 
age to engine. For large engines an auxiliary pump is employed to 
replace crank case compression. 

FOUR-CYCLE ADVANTAGES. 

Better explosion control; more economical; compression not de¬ 
pendent upon tightness of any part except valves and piston rings. 

FOUR-CYCLE DISADVANTAGES. 

Cylinder volume and weight per unit of power greater; multi¬ 
plication of parts, especially valves, valve gear, cams, countershaft, 
etc., with loss of power if any valves are not gas tight. 


GASOLINE ENGINES. 


19 


The four-cycle engine seems to lose in simplicity by comparison 
with the two cycle, but it is in far more common use. The two-cycle 
engine has given way to the four-cycle type for airplane use. 

Although the two-cycle engine receives twice as many impulses 
per revolution as the four cycle, it must not be concluded from this 
that, for the same cylinder dimensions, the two cycle has twice the 
power. In the four-cycle type the impulse, due to expansion, is 
carried throughout nearly the entire stroke, whereas in the two-cycle 
type the exhaust valve opens much earlier and the impulse only 
lasts about five-eighths of the stroke. 


MEASURING PARTS OF REVOLUTIONS—DEGREE MEASURE. 

In order to a little more fully explain the principles involved in 
the gasoline engine it will be necessary to refer to a partial revolu¬ 
tion of the crank shaft, such as a half revolution, a quarter revolu¬ 
tion, or some smaller fractional turn. To state definitely what part 
of a turn it is desired to express recourse is made to what is called 
u sexagesimal ” or “ degree measure.” 

The degree is the one-three hundred and sixtieth part of the cir¬ 
cumference of a circle. If a circle had a circumference of 360 inches 
in length then each inch would represent a degree. A quarter of a 
circle would be 90°; a half circle 180°, etc. The word “ degree ” 
originates with the ancients of early Babylon; these ancients indi¬ 
cating that the earth required 360 days to pass its circular orbit 
around the sun; hence the division of a circle into 360 parts. If a 
crank shaft is given one quarter turn it is said to have turned 
“ through an effort of 90°.” A turning effort of the crank shaft 
of 360° would represent one revolution; two revolutions measured 
in succession would be 720°. The gasoline engine cycle is, therefore, 
720° of crank-turning effort in length. 


MORE EXACT INFORMATION ON THE STROKES OF A CYCLE. 


Thus far it has been indicated that the power stroke, exhaust 
stroke, etc., of the gasoline-engine cycle are each approximately 
one-half revolution in length. In actual practice on the motor this 
is not true; the strokes vary more or less from 180°, according to the 
design ideas expressed in the construction of the motor. For exam¬ 
ple, one well-known motor has a cycle whose strokes are as follows: 

Crank-turning effort. 

Intake stroke_215° 

Compression stroke- 135° 

Power stroke-130° 

Exhaust stroke- 240° 

The complete cycle-.- 720° 







20 


GASOLINE ENGINES. 


It will be noticed that the power stroke is less than one piston 
stroke of 180°. The exhaust stroke, on the other hand, is much 
longer than a piston stroke (it being understood that a piston stroke 
has the definite length of 180°). These events of varying length are 
called “cycle strokes” in order to distinguish them from the so- 
called “piston strokes” of the motor. Motors will differ in their 
design, depending upon many little details of construction, and 
will differ as to the exact lengths of the cycle strokes. For example, 
another well-known motor, instead of having a power stroke of 130°, 
has a power stroke of 135°; while still another has a power stroke of 
128°. In fact, each designer in planning out the details of a motor 
is very likely to express a difference in cycle-stroke lengths from 
that of any other motor. The reason for these varying lengths will 
be fully explained in the subsequent chapter. 

COMPARISON OF AIR AND WATER COOLED TYPES. 

The metal weight of the water-cooled motor is about one and one- 
half times that of the air-cooled motor. The consumption of the 
air-cooled motor is approximately twice that of the water-cooled 
motor. 

The air-cooled motor can run at higher temperatures than the 
water-cooled motor; and as there is less difference in temperatures 
between the cylinder walls and the heat of explosion, there is less 
loss of heat by radiation, or less “ heat loss.” The air-cooled system 
is simpler and lighter than the water cooled. It has no freezing 
troubles in winter, but as it runs hotter, this means lower compres¬ 
sion must be used, because on hot days, with the atmospheric tem¬ 
perature higher, the cylinder temperature will run higher, and the 
cylinder temperature may be several degrees hotter. This means 
the designer has no upper limit on his temperature and must make 
the combustion chamber of a size which will give a compression 
suited to the highest likely temperature. This means comparatively 
low compression. With water-cooled motors, as the water tempera¬ 
ture reaches 212 degrees (boiling point of water), the water is 
rapidly lost. Therefore there is quite a definite upper limit to the 
temperature at which this type of motor can be run, and this means 
that a comparatively high compression can be used and better fuel 
economy obtained. 

A motor which has excessive compression' will run well for a short 
time, then will lose power from preignition. Airplane motors are 
particularly likely to be troubled this way, as they are run continu¬ 
ously on full throttle, which also means full compression. 

A disadvantage in connection with the cooling of a rotary engine 
is that the leading edge of the cylinder is much cooler than the 


GASOLINE ENGINES. 


21 


trailing edge; this gives a difference of temperature around the 
cylinder walls, producing a distorted cylinder, and sc necessitating 
the use of a special type of piston ring which is known as “ obdurator 
ring.” This type of ring is fitted because it will easily follow the 
distortion of the cylinder, thus keeping the piston compression tight 
in the cylinder. This ring is a constant source of trouble. 

Summing up: The advantage of the water-cooled engine is longer 
life and a higher thermal efficiency, due to a smaller loss of work 
from overheating. The disadvantages attached to a water-cooled 
engine from an aeronautical point of view are that it is much heavier 
and the possibility of trouble due to freezing of water. 


CHAPTER III. 

THE VARYING LENGTHS OF CYCLE STROKES. 


PECULIAR RELATION BETWEEN PISTON AND CRANK-PIN TRAVEL. 

The definition of a piston stroke has already been given. Its pro¬ 
portionate travel, however, in connection with equal movements of 
the crank pin upon the crank path circle has not been explained, yet 
this explanation is necessary to fully understand the “ why ” of the 
cycle stroke lengths. Note the diagram below: Z? is a crank path 
circle with a cylinder center line drawn through it. Naturally, as 
the crank pin travels over the crank path circle D , the piston will 
move to and fro along the cylinder center line, as indicated by the 
bracket labeled “ piston stroke.” Between top center “ 0 ” and bottom 



Note.—“ A ” for crank pin position and “ B ” for piston position in the figure, 
the exact setting at 130°. A comparison of the power stroke to a full piston 
stroke is thus easily made. The length of the power stroke is so much greater 
than would be imagined that it hardly seems credible. 

center “ 8 ” a number of equal distances upon the crank path circle 
have been stepped off. There are eight of these distances between the 
centers, each being equal to 22^°. Now, when the crank pin is at 
point 1, the piston will occupy a relative position as point 1' in the 
piston stroke, and so on, for the following points, 2, 3, 4, etc., to 
bottom center. It becomes apparent in observing the movement of 
the piston, as compared with the equal crank pin movement, that 
the piston moves large distances between points 2' and 5', and small 
distances between 0' and 2', as well as between 5' and 8'. As a 
result, when the crank pin is at 90°, point 4, the piston is more than 
22 







GASOLINE ENGINES. 


23 


half way down in its stroke. If the average crank pin travel of a 
power stroke is 130° less than three-quarters the distance from top to 
bottom center, then the corresponding travel of the piston will be 
approximately nine-tenths a full piston stroke. 

EARLY AND LATE VALVE OPENINGS. 

The two main observation points on the crank path circle are the 
two centers, top center and bottom center. If any mechanical event 
occurs when the crank pin is in the neighborhood of one of these 
centers, it is said to occur “ early ” or “ late,” according to whether 
it happens before the crank pin reaches the center, or after. For 
example, suppose the power stroke to be approximately 130° long 
and to start at top center. The exhaust valve would then open 130° 
after top center, or, in the usual shop language, “ 50° early,” meaning 
50° before reaching bottom center. Now, again, if the exhaust stroke 
was 240° long, the exhaust valve would close “ 10 ° late,” or 10 ° past 
top center. Thus, one mechanic may say to another, “ How late is 
the exhaust closing on this motor? ” The reply would give him the 
exact timing he sought, as it would be expected of him to know which 
of the two common centers were referred to. 


WHY THE EXHAUST VALVE OPENS EARLY vBEFORE BOTTOM CENTER). 


As we enter into a simple discussion of the exhaust valve timing, it 
is best that first a conception of motor speed be obtained. A little 
thought applied to the subject will recall that the slowest propeller 
speeds are in the neighborhood of 1,200 revolutions per minute. The 
fastest speeds are around 1,700 revolutions per minute. These speeds, 
of course, are those attendant upon maximum motor effort. It is cus¬ 
tomary. in the cases of all motor speeds less than 1,700 revolutions per 
minute, to “ direct connect ” the propeller to the crank shaft of the 
motor. So, if the average propeller speeds lie between 1,200 and 1,700 
revolutions per minute, then, so do the average motor speeds. Let us 
select for illustration a motor speed of 1,400 revolutions per minute. 
Then, since there are 60 seconds to one minute— 


1,400 divided by 60 gives 23.333 or 23.333 turns a second. 

1 piston stroke equals one-half revolution. 23.333 revolutions 


per second means one revolution is made in 23^333 


part of a second. 


One-half of 23 333 58 46.666 ° r 46.666 part of a second - 


Thus we find that a piston stroke is completed in one forty-sixth 
to one forty-seventh of a second, a very short period of time. 






24 


GASOLINE ENGINES. 


In dealing with the gasoline engine it is a good plan to bear in 
mind that nothing can happen “ instantaneously. 5 ’ though certain 
happenings may occur with startling rapidity; a time interval must 
always be allowed to u cover ” the performance. A good illustration 
of this is the duck hunter. When a hunter fires at a duck on the 
wing, passing by him, he must aim a certain distance ahead of the 
duck. If he aims at the duck, and the gun shoots true, he will miss 
it to a certainty, unless the duck is very close to the gun. The reason 
for this is that a time element is required for the trigger hammer to 
fall on the cap, the powder to burn, and the shot to travel the distance 
from the gun to the position of the duck. During these happenings 
the duck is moving. This is also true of many instances of the gaso¬ 
line engine; an “ advanced timing ” is required, that the happenings 
may be in keeping with the great piston speeds. 

If all downward strokes of the piston were accompanied by a posi¬ 
tive pressure upon the piston head, and all upward strokes by a nega¬ 
tive pressure, the engine conditions would be ideal. Such conditions 
are impossible; the best that can be hoped for, especially in consider¬ 
ing the power and exhaust strokes, is that the pressure during the 
power stroke be as great a positive pressure possible to be obtained 
from the fuel, and that the exhaust stroke be as mild as a positive 
pressure consistent with the ejection of the burnt gases. The exhaust 
stroke, then, at the best will have a retardant effect upon the piston. 
It will further follow, that whenever the pressure is positive within 
the combustion chamber of the cylinder, it will voluntarily move out 
to the atmosphere if an escape passage is provided. The outrush will 
depend on how positive the pressure is. 

In referring to the travel of the piston downward during the 
power stroke (see Fig. 1), the major movement has occurred prior 
to point 6, approximately 135°. The final 45° to bottom center re¬ 
sults in but slight piston movement. The operative pressure of the 
gases forcing down the piston has been greatly reduced by the time 
point 6 is reached. The piston has already permitted an extensive 
enlargement of the combustion chamber. Why, then, not utilize the 
opportunity afforded by the final 45° or so to effect a voluntary un¬ 
loading of any positive pressure within the combustion chamber? 
This would bring the pressure more nearly normal at bottom center, 
and the piston could then start its next upward stroke with less re¬ 
tardation. 

Summing up the above, the following defined reasons can be as¬ 
signed to an early opening of the exhaust: 

(a) When the crank pin reaches the neighborhood of 45-50° be¬ 
fore bottom center, the piston has performed the major part of the 
first piston stroke and has allowed for all reasonable expansion of 
the power gases. 


GASOLINE ENGINES. 


25 


(b) The great rapidity of piston movement is such that unless an 
“ early exhaust ” is started, bottom center will be passed with an un¬ 
desirable pressure still reposing in the combustion chamber space. 

(c) After passing the neighborhood of 45-50° before bottom cen¬ 
ter, “the cranking angle” is such that the connecting rod can no 
longer impart rotary motion to the cranks to advantage. (See 
angle C, Fig. 1.) 

( d ) The terminal pressure of the power stroke, though only a frac¬ 
tion of the beginning pressure, is still highly positive (about 40 
pounds to the square inch) at 45-50° before bottom center and will 
voluntarily rush out if the exhaust valve is opened, thus effecting a 
large volume of the exhaust without piston retardation. 

( e ) The power gases and exhaust gases are extremely hot, and the 
longer they repose in the combustion chamber space the more heat 
they radiate to exposed surfaces, thus, breaking down lubricants and 
interfering with proper cooling. The earlier the exhaust is started 
the less this trouble. 


THE EXHAUST STROKE. 

The exhaust stroke is the longest cycle stroke of the four. It is 
much over the length of the complete piston stroke, and therefore 
must not at any time be imagined as an upsweep of the piston 
merely. Naturally, the air-cooled motors have longer exhaust strokes 
than the water cooled, as the average air-cooled motor opens its ex¬ 
haust valves earlier. In fact, some air-cooled motors start their ex¬ 
hausts as early as 85° before bottom center. 

It might be said here, just to convey a comparative idea, that the 
average water-cooled motor starts the exhaust 50° before bottom 
center, while the average air-cooled 70° before bottom center. 

That part of the exhaust stroke occurring prior to the passing of 
bottom center by the crank pin is usually termed the “ voluntary por¬ 
tion,” while that part existing between bottom center and top center 
is usually termed the “ forced portion.” And, as will soon be seen, 
the exhaust continues past top center some 10° on the average, form¬ 
ing a portion usually termed the “ second voluntary portion.” 

WHY THE EXHAUST VALVE CLOSES AFTER TOP CENTER. 

If the student will pick up a detached cylinder, turn the open bore 
toward him, and glance down the bore, he will observe that the valve 
heads are comparatively small. Yet it is through these valve ports 
that all gases must find ingress and egress to the cylinder. The 
engine designer is always studying methods of making the valve 
ports as large as the practicalities in construction will permit him. 


26 


GASOLINE ENGINES. 


It is observed, however, that during the upward movement of the 
piston through the exhaust stroke its velocity becomes so great that 
the stale gases can not hope to escape in the proper proportion. 
Thus, the piston reaches the top of its travel with a slight congestion 
of the exhaust gas ahead of it confined in the combustion chamber. 
The pressure is, therefore, slightly positive at top center. To close 
the exhaust valve now would be confining burnt gases capable of 
voluntary ejectment. The natural place for the exhaust valve to 
close would be upon a normal pressure, which is likely to be reached 
in the neighborhood of 10° past top center. 

In the block testing of motors it has been determined that the 
exact timing of the exhaust closing is a very critical setting, a varia¬ 
tion of 5° too early or 5° too late, greatly affecting the horsepower 
delivered by the motor at full-load high speeds. 

If irregularities are experienced in the valve-operating mechanism 
of a motor, and these irregularities affect the valve timing, it is much 
better to introduce them, if possible, upon the opening rather than 
the closing. For example, suppose the exhaust on a given motor 
should open 50° early, bottom center, and close 10° late, top center, 
a total exhaust-opening dwell of 240°. Instead, it is found that 
the best possible opening dwell to be had is only 235°. Question: 
Shall we let the valve open on time or close on time? Answer: It 
should open 5° late and close on time. 

THE OPENING OF THE INLET VALVE. 

Up to a very recent date aviation motors differed widely in respect 
to the opening of the inlet valve upon comparison of their cycles. 
Of course, the different speeds and different valve sizes and designs 
had a great deal to do with the variation. Recent American practice 
places the inlet opening within a 5° range of the exhaust closing, 
while generally speaking the inlet opens as the exhaust closes. 

When the inlet valve opens before the exhaust valve has fully 
closed, the valves are said to have a “ positive overlap ” in their tim¬ 
ing. Also when the inlet valve opens before top center this term is 
frequently applied, as, so one would suppose, any period prior to 
top center rightfully belongs to the exhaust stroke. 

When the inlet valve opens exactly as the exhaust closes the valves 
are said to be “ simultaneously timed.” 

When, as in many cases, the inlet valve opens a few degrees after 
the exhaust has closed, a “ valve timing interval ” is said to exist. 

THE PRINCIPLE ATTRIBUTED TO THE VALVE TIMING OVERLAP. 

The opening of the inlet valve a noticeable time prior to the ex¬ 
haust closing is only done upon very high-speed motors; motors turn- 


GASOLINE ENGINES. 


27 


ing over faster than 1,800 revolutions per minute as a rule. This 
principle of valve timing is based upon the fact that a volume of gas 
inclosed within a pipe has weight and therefore inertia. 

Each unit volume of gas or air has weight, just as any other sub¬ 
stances, whether fluid or solid, would be expected to have weight. If, 
then, a volume of gas is started down a tube or pipe, the moving mass, 
due to its weight, will require a force to stop it. Due to the extreme 
lightness of most gases, they must gain considerable velocity before 
their momentum represents any appreciable inertia; that is, requires 
any appreciable force to bring them to an abrupt full stop. With 
the same idea in mind, a given volume of gas at rest in a pipe will 
resist a sudden start, all due to its weight, or, properly speaking, its 
inertia. 

Now, if the high-speed motor has a well-developed exhaust pipe or 
manifold, the exhaust gases may attain an enormous velocity through 
it, thus possessing considerable inertia. This will frequently happen 
when the exhaust pipe is only 4 to 8 inches long. It then follows 
that the piston reaches top center during the exhaust stroke; the 
escaping exhaust gases, due to their inertia, will exert a “drag” 
or suction action on the contents of the combustion chamber, result¬ 
ing in the rapid lowering of combustion-chamber pressures. Pro¬ 
vided the exhaust valve opens early enough, giving the exhaust a 
good voluntary start, the action described above will bring the pres¬ 
sure within the combustion chamber slightly below normal at top 
center. If then the inlet valve be opened, this slight negative pres¬ 
sure will help overcome the inertia of the stationary intake gas. 

The plan of the positive valve overlap is considered by many a 
dangerous backfire hazard. For if the motor be not fully up to speed 
and quick operation be given the throttle, it is almost certain to back¬ 
fire, with the usual danger of setting fire to the carburetor. Also 
in the event of failures of the ignition apparatus or other motor 
assemblies the backfire seems more likely to occur. 

The simultaneous valve timing or an interval of 3 to 5 degrees 
seems to be best thought of by the average motor operators, and most 
of our American training planes have been characterized by this 
type of motor. The new Liberty expresses this characteristic of 
valve timing. 

WHY THE INLET VALVE CLOSES. 

It is well to remark here that the intake air passes a rather tor¬ 
tuous passage in reaching the inlet valve port. Every turn the col¬ 
umn of air makes means added resistance. First, are a number of 
turns through the preheater stove; second, down the supply pipe to 
the carburetor scoop; third, through the carburetor with its Venturi 


28 


GASOLINE ENGINES. 


choke; fourth, up a bending manifold; and, lastly, through the twists 
and turns of passing the inlet valve. All of this is resistance. Now, 
keeping in mind the above resistance, remember the rapid down 
movement of the piston and it should not be hard to conceive that 
bottom center is reached with the pressure within the combustion 
chamber still negative. As long as this pressure remains below the 
normal the new charge will continue to flow in. During the next 
30° the piston will move up but little. The inlet valve is therefore 
allowed to remain open until by reason of the combined results of 
the continued inflow of the new gas and a slight uptravel of the 
piston the internal pressure is approximately equal to the atmosphere. 
This will be some 35° or 40° past bottom center on the average motor. 


CHAPTER IV. 

TYPES OF MOTORS. 


CLASSIFICATION. 

Aero engines may be divided into two main classes, rotary and 
stationary. Engines ol the stationary class may be further divided 
into the radial, vertical, and vee types, all of which may be air or 
water cooled. 

RADIAL ENGINES. 

In this type of engine the cylinders are placed around a single 
crank pin in the same manner as the spokes of a wheel are placed 
around a hub. This type gives a very short and rigid shaft. The 
crank case also is a strong barrel type of case not divided in the 
center. Collection of oil is kept out of the lower cylinders by keep¬ 
ing the crank case pumped out. This type of motor is hard to 
inclose in a stream-line hood. 

In comparison with other stationary engines, the radial engine 
has the advantage of very low overall length. Engines of this type 
are not at present as popular as they were formerly. 

ROTARY ENGINES. 

Cylinders in this type of engine are arranged the same as the 
radial but operate differently. In this case the cylinders revolve 
and the crank shaft remains stationary. The crank shaft is bolted 
to the airplane and the propeller is attached to the crank case, just 
the reverse of the practice with stationary cylinder motors. 

The rotary engines combining the principles which tend to lighten 
the weight, as in radial motors, with an extremely light all-steel con¬ 
struction, are the lightest motors in use. 

DISADVANTAGES. 

They consume so much fuel and oil that if w r e weight them with 
fuel for a long flight they are heavier than the stationary cylinder 
type; therefore they can only be used to advantage in short flights. 
The reason they use so much fuel is that, as they are air cooled they 
have comparatively low compression; and also that they require 
between 10 and 12 per cent of the power they develop to rotate the 
engine, or drive the cylinders through the air. 


29 


30 


GASOLINE ENGINES. 


VEE TYPE. 

The vee type engine is a development of the foreign six-cylinder 
vertical type, resulting from the demand for increased power. By 
the introduction of the vee principle, the number of cylinders is 
doubled without increasing the length of the cylinder and with little 
alteration to the effective width. In practice six-cylinder vertical 
engines are built in horsepowers up to 200, while 12-cylinder vee 
engines are built to give some 360 horsepower. 

VERTICAL TYPE. 

In a vertical engine the cylinders are arranged in a row, standing 
vertical. Motors with one, two, three, four, or six cylinders are 
nearly always made in this type. If we attempted to make eight- 
cylinder motors in this type the cylinders in line would make a 
very long motor, which would take up too much room in the air¬ 
plane. The airplane fuselage, and also the crank case, would have 
to be made quite heavy in order to be stiff enough. Eight and twelve 
cylinder engines can be built w T ith the cylinders in line or vertical 
for marine purposes, but are not likely to be built in this way for 
airplanes. Suppose we built an eight-cylinder motor and arranged 
the cylinder in two rows of four, side by side. By doing this we 
cut in half the length and width of the crank shaft and crank case, 
thus making the motor lighter, more compact, and rigid. 

ROTARY AND STATIONARY ENGINES. 

In comparing the rotary with the stationary engine it is not pos¬ 
sible to lay down hard and fast rules. Broadly speaking, it may be 
said that the field of the rotary is in the propulsion of very fast 
small machines of the scout type, while the stationary engine is 
now employed in practice in every class of machine, but particularly 
in machines used for making long flights. The rotary is the lightest 
type of engine for a given horsepower, but it is wasteful in lubri¬ 
cating oil and is relatively inefficient in fuel consumption. It there¬ 
fore pays to use this class of engine where flights of long duration 
only are contemplated, so that the advantages of low weight of engine 
per horsepower will not be counterbalanced by the relatively large 
amount of fuel and oil to be carried. The rotary engine is also 
regarded as being less reliable than the stationary, and engines of 
this type require more frequent overhauling. The average rotary 
engine must be taken down after 40 hours of running, while most 
stationary engines can be relied to run some 100 hours without over¬ 
hauling. From this point of view, however, and in the rotary engine 
in particular, a very great deal depends upon the design of the 


GASOLINE ENGINES. 


31 


engine and upon the materials and workmanship employed in its 
manufacture, overhauling, and adjustment. Three out of four of 
the principal rotary engines have steel rollers without lining, and 
the pistons are fitted with obturator rings which in the hands of 
careless or unskilled mechanics would prove a source of perpetual 
trouble. 

TYPES OF CYLINDERS. 

There are several types of cylinders used with gas-engine construc¬ 
tion, named mostly after the shape of their cylinder head or the 
location of the valve: 

(1) Valve-in-the-head cylinder. 

(2) u L”-head cylinder. 

(3) “ T ’’-head cylinder. 

(4) “ I ’’-head cylinder. 

(5) Angular-valve-in-head cylinder. 

(6) Four-valve-in-head cylinder. 

For airplane use three of these are most common—the angular- 
valve-in-head type, the valve-in-the-head type, and the four-valve-in¬ 
head type. 

The angular-valve-in-head cylinder has the valves set at an angle 
of about 45 degrees, with the heads of the valves seating in the cylin¬ 
der, and the valves are operated by push rods and rocker arms. 

The valve-in-head type of cylinder has the valves placed either 
vertically or at a slight angle apart, with the heads of the valves 
seating in the head of the cylinder. This gives a combustion chamber 
which has no pockets, but the valves must be operated by push rods 
and rocker arms or else the cam shaft must be placed along the top 
of the cylinder with small rocker arms operating the valves direct 
from the cam. In this case the cam shaft is held in an oil-tight 
housing. 

The more square inches of surface exposed to the head in the com¬ 
bustion chamber, the more heat will be absorbed and the greater will 
be the heat loss. Therefore, the combustion chamber which exposes 
the least surface to the heat will cause the least heat loss. This also 
works another way; the cylinder exposing the least surface to the 
heat in the combustion chamber will require the least cooling. This 
may mean reducing weight in the cooling system. In the case of 
high-speed motors it is extremely important to have very direct gas 
passages. In other words, the gas should be able to go direct into the 
cylinder and out without having to flow around corners or through 
pockets in the sides of the cylinder. 

The four-valve-in-head type of cylinder gives the most direct gas 
passage, which means such a cylinder can receive a more complete 
charge of gas at high speed, and the exhaust gases can be more com¬ 
pletely expelled or scavenged. 


CHAPTER Y. 


VALVES AND VALVE OPERATING MACHINERY. 

THE POPPET VALVE. 

Much depends upon the valves of the gasoline motor; in fact, a 
great deal of the motor efficiency lies in the condition and operating 
accuracy of the entire valve mechanism. Since the valves are used 
to admit the gases to the cylinder and permit their ejection at 
“ timed ” intervals, they are the mechanical expression of the actual 
cycle. 

Throughout a long period of mechanical endeavor the accomplished 
motor attendant has prided himself upon obtaining an accurate set- 



ting of the valves upon any motor assigned to him. This fact should 
emphasize the importance attached to valves as well as to make the 
beginner realize how careful he must be of the most minor adjust¬ 
ments to this part of the motor mechanism. 

There are many kinds and types of valves, but the most common 
type used in the aviation motor is the “ poppet ” or “ mushroom ” 
valve, so called on account of its action and shape. It preeminently 
stands first in point of use amongst all other types. The figure 
above shows the usual design taken by this valve. It has two main 
32 





























GASOLINE ENGINES. 


33 


parts, the “ head ” and the “ stem..” Of the two, the head is the valve 
proper, while the stem is the valve lifter, and, as will be noted later, 
much depends upon the shape and sizes of these parts. 

MATERIAL AND THE MAKING OF POPPET VALVES. 

The chief considerations in the manufacture of valves is to make 
them accurate, cheap, and of heat-resisting material. One of the 
main endurances that the valve must meet in operating service is 
the high temperatures of the gases passing by or over them. These 
gases are far above the fusion point of the material of which the 
valves are made, and if it were not for the fact that much of the heat 
taken on by the valves was almost immediately given up to water- 
cooled portions of the cylinders the valves would soon become use¬ 
less. As it is, the face of the valves, especially ‘the exhaust valves, 
rapidly become “ pitted ” or “ burned ” or “ carbonized.” It has 
been found that valves made of certain steels resist the heat action 
better than others. Naturally, it would be thought that just such a 
type would be the best, but many engineers object to them on account 
of other disadvantages that are introduced, one of which is extreme 
hardness after little service, accompanied by possible brittleness. 

Of all the different ways valves are made we will, for simplicity, 
sum up the process in the following : 

(a) The two-piece method .—The valve head and stem are made 
separately and by the modern ingenuity of the electric welder or the 
gas welder the two pieces are jointed. They are then machined and 
grinder finished. This process makes possible the making of the 
head of one type of steel and the stem of another; thus the head may 
be made of costly heat-resisting steel and the stem of less costly wear- 
resisting steel. Nickel-steel is usually used for the head. The com¬ 
bination is desirable, but the common fault of so many of the valves 
of this process is that the fillet immediately under the head is fre¬ 
quently too small. 

(h) The one-piece drop-forge method .—A so-called valve blank is 
formed by the drop-forge method in this process. Nickel-steel is 
generally used. This type of steel combined with the method permits 
a very light weight and strong valve to be made. The nickel-steel 
will withstand considerable heat and will not warp or pit. 

However, of all possibilities of heat-resisting material for valves 
actually doing what the term implies, probably the most earnestly 
advocated by the aviation engineers is the “tungsten” steel valve. 
This seems to be the best material found so far for valves. A valve 
made of it is very hard and will not pit as so many valves do. It can 
be made very light, as it is free from warping, and the weight of 


34 


GASOLINE ENGINES. 


valves in high-speed motors is a big item. Generally the tungsten 
steel valve is used with a very narrow seat, possible only by reason 
of the hardness of its face, giving a free, clear opening, and allowing 
the passage of gases with least possible resistance. One of the com¬ 
mon trade names for a tungsten steel valve is the “Rich” valve. The 
name is usually imprinted faintly on the top of the valve. 

NUMBER AND MOUNTING OF VALVES. 

It is the common practice to use one intake valve and one exhaust 
valve to the cylinder, generally seating against the cylinder metal. 
Occasional motors are encountered where two exhaust valves and 
two intake valves are used per cylinder. This latter practice is to 
obtain more valve opening area for a given sized bore cylinder, as it 
is always desirous to afford as much valve opening area as possible 
in a high-speed motor. Duplicating valves in this fashion greatly 
increases the labor and cost of maintenance. It is best that the 
student observes carefully the valve setting in the head of a cylinder 
by taking a detached cylinder in his hands, glancing down the bore 
and examining the valve ports of the cylinder. He will readily see 
that there is a decided limit to the size possible for designing the 
exhaust and intake for a given bore cylinder and yet leave sufficient 
structural metal in the cylinder head for strength and water jacket 
space. By examining two or three such cylinders from different 
make motors he will notice how, in some cases, the designer has 
skillfully shaped the combustion chamber so as to permit the placing 
of larger valves than would otherwise be possible. 

When a valve is mounted in the cylinder, as shown in the position 
of Figure 2, it is said to be mounted “ vertically head up,” the center 
line of the valve stem being truly vertical and not considered so when 
parallel to the cjdinder center line. 

When mounted exactly the other way up from the above it is said 
to be mounted u vertically inverted.” With the same idea, a valve 
may have “ horizonal ” mounting or “ oblique mounting. Most 
“ valve-in-the-hrad ” cylinders have the inverted vertical or oblique 
mounting. Most all “ L ’’-head cylinders used the vertical head-up 
mounting. 


MISFIT VALVES AND FAULTS OF SEATING. 

Valve efficiency depends upon three things—a gas-tight meeting 
between face and seat, a correct width of opening, and a correct 
timing of opening and closing. At present we will discuss largely 
the first two. It is hopeless to obtain the power out of a motor if 
the valves leak to a small degree. They must be absolutely gas tight. 
A small leak is only a small leak for the moment; the hot gases 


GASOLINE ENGINES. 


35 


squeezing through a small leak have the ability to rapidly burn and 
cut away the steel to such an extent that the action of the cylinder 
soon becomes useless. With aviation motors this burning away of a 
leaky valve is much faster than will ever occur in an automobile 





motor, on account of the high speed and the full loading of the 
former. 

The valve face should always be wider than the seat; the seat is 
usually 75 per cent to 80 per cent width. If the seat is too narrow 
and the face a soft metal, then the face will become rapidly chan¬ 
neled by the seat. The seat impresses itself in the face. This will 



Fig. 7. 


happen in course of time, anyhow, and is bound to happen if the 
valve is overground. It has already been stated that a tungsten steel 
valve has a very hard face, the tungsten steel becoming very hard 
when subjected for a short time to the operating heat of the motor. 























































36 


GASOLINE ENGINES. 


Such a hard valve face is liable to deform the seat as rapidly as the 
face channels, therefore, when tungsten steel valves are used, keep as 
careful a watch on the valve seats as on the valve face. 

Much care is to be taken when placing new valves in a motor to 
see that the valve does not seat too “ deep ” or too “ high.” The 
valve face must cover the seat. If the mechanic is careless in the 
assembly, he stands a chance of mixing the valves belonging to the 
various cylinders and thereby placing the wrong valves upon the 



wrong seats. The face angles, in such a mishap, may correspond, 
but the valves may seat too deep or too high, with the result that the 
seating area is incapable of withstanding the high pressures and the 
valves burn out. 


GRINDING VALVES. 

When a valve starts to leak the cause for the leakage is generally 
the failure of the face to meet the seat accurately at all points. For¬ 
eign matter, such as carbon, etc., may have lodged on the seating 



























GASOLINE ENGINES. 


37 


area, holding the valve open long enough to become slightly burned. 
Or the valve may have been held open by improper adjustment of 
the valve apparatus. However, when the leakage is detected, the 
valve must be “ ground.” This grinding procedure is best given in 
outlined steps: 

(a) Remove the valve from its seat. This is usually done by re¬ 
moving the cylinder from the motor and releasing the valve spring 
retainer washer at end of stem, then sliding the valve down into the 
bore of the cylinder. Motors with removable heads, or of the “ L” 
head type, do not require the removal of the cylinder from the motor. 

(b) Carefully scrape all carbon from valve head, edge, face, and 
under head on fillet. This must be done with a piece of brass or 
material softer than the steel of the valve so as not to scratch it. 

( c) Carefully scrape all carbon from valve chamber and around 
seat. 

( d ) Wipe off valve face and seat and inspect both for any irregu¬ 
larities, such as ridges, burned spots, pits, etc. 

( e ) Apply to valve face a little coarse grinding paste. Paste 
must not be too thick nor so thin that there is danger of it drip¬ 
ping off. 

(/) Drop valve in on seat and apply holding chuck to end of 
stem. 

(g) Methodically grind valve by making, say, a half left turn, 
raising valve slightly from seat, then a three-quarter right turn, etc., 
avoiding direct rotation of valve on seat which will cause the grind¬ 
ing compound to make parallel ridges on face and seat. (Note that 
this process is distinctly different from the usual grinding of auto¬ 
mobile motor valves, which are heavier with wider faces and seats. 
This latter type valve is generally milled on head for reception of a 
screw driver bit and during the grinding process much weight is 
placed on it at each turn, then it is chattered on the seat violently 
at each pick-up.) Rough handling of an aviation motor valve, 
coupled with the frequent grinding it must receive in comparison 
with the automobile valve, would soon depress it in its seat. (See 
drawing above for depressed valve seat.) 

(A) Frequently wash compound off and apply fresh. At first 
wash off, note valve face critically; see if a dull band is developing 
around face evenly where contact with seat is made. If dull gray 
band is not even, either valve face or seat is not true and the fault 
must be located. 

(i) When dull gray band is distinct around face (don’t attempt 
to grind out all pits unless a series of them form a path across face), 
apply some of the finishing compound and grind with a lighter and 
lighter pressure on valve. 


38 ' 


GASOLINE- ENGINES. 


(j) Wash off all traces of the compound from seat and valve. 
Apply a little machine oil and “polish 55 face and seat with the 
usual grinding “swing. 55 

(7c) Again clean and test with gasoline; that is, place valve on 
seat and pour a little gasoline into valve chamber. Examine from 



cylinder interior for any trace of leakage of gasoline. Wipe off if 
O. K. and oil slightly. 

So many mechanics look upon grinding as a simple easy-go- 
lucky process and are prone to be very careless and insist that they 
are expert at the job. This fact makes it very hard to enforce ex¬ 
acting care. 




































GASOLINE ENGINES. 


39 


the valve operating mechanism. 

The character of the valve operating machinery differs widely on 
the various makes of motors. This difference may be roughly 
classed under three headings: First, the valve-in-the-head type with 
the rocker arm push rod system; second, the valve-in-the-head with 
overhead cam shaft; and third, the “ L ” head type. 

WEAR IN THE VALVE OPERATING MECHANISM. 

Naturally, with as many small rapidly reciprocating parts as in 
the valve operating mechanism upon a motor, wear will take place 
quickly on some and less quickly on others. This wear affects the 
motor in two ways, namely: 

(a) Wear tending to merely “ loosen up ” the parts. 

(b) Wear tending to mis-time the valves in their opening and 
closing, as well as their lift. 

The second item is by far the more important and is emphasized 
in a pamphlet on Valve-Timing. The first may be said to be a mat¬ 
ter of mechanical judgment, and when a part, such as a rocker arm 
pin, becomes noticeably loose, it should be replaced with a larger pin 
and the parts affected renewed. 

AIR LEAKAGE AROUND VALVE STEMS. 

It is still further to be pointed out in the discussion on valves 
that a valve itself may show wear at other than just the face. The 
stem may wear where it rubs the stem guide, and, of course, if the 
stem wears, so does the stem guide. Usually the stem guide in large 
motors is a bushing, a pressed fit to the cylinder head metal. But¬ 
in the Smailer varieties, it is "the cylinder metal itself. Short valve 
stem guides wear quickly and are hard upon the life of the valve 
stems. The result is that stems and guides do not necessarily last 
the life of the average cylinder and will become troublesome, in 
that they allow gases or air to pass to and fro along the valve 
stem between valve chamber and the atmosphere. In the case of 
the exhaust valve, the leakage of exhaust gas down the stem tends 
to overheat the valve by keeping the stem too hot, as a good bit of 
the cooling of the exhaust valve is dependent upon the conduction 
of heat from valve head through fillet to stem. From the stem it 
is given up to the air or cooler guide. In the case of the inlet 
valve, it allows atmospheric air to pass along the stem and into 
the valve chamber, thus rarefying the mixture. It is often neces¬ 
sary in the set up of the motor to select a new valve with an over¬ 
sized stem, as special “spares” are made for motors which have 
experienced service. In the type of motor with the removable 


40 


GASOLINE ENGINES. 

guide, all that is necessary is to replace the guide. It might be 
well to caution that the valve stem on the other hand must not fit 
the guide too tightly. A running fit ample for heat expansion of 
stem must be assured. 


VALVE SPRINGS. 

It would be rather short-sighted judgment to express exacting care 
regarding the fit of a valve and then mount it with a spring that 
would not at all speeds return it promptly to its seat. Yet mechanic 
after mechanic has been frequently found guilty of just such mis- 
judgment. This is because the average man is not aware of the 
rapidity w r ith which a valve spring will weaken under high-speed 
duty. Then, besides, different motors use different spring tensions 
for their valves, and it becomes hard to lay down a good rule for 
detecting a weak set of springs, and therefore too frequently weak 
springs pass unnoticed. 

A high-speed motor requires a strong valve spring to return one 
of its valves to a seat at high speed. A low-speed motor does not 
require so strong a spring. The more a valve weighs the harder 
it will be to promptly return it to its seat. On the other hand, too 
strong a spring will cause the valve face to channel or ridge quickly 
and further will cause unnecessary wear upon the valve-operating 
mechanism. If high efficiency is to be had at high speed, the right 
spring tension must be secured with light-weight valves.. 

The spring tension must be such that the exhaust valve can not 
be sucked slightly off its seat during the intake stroke of the motor, 
a condition hard to detect upon the operating motor. Most motors 
have interchangeable valve springs—that is, an exhaust-valve spring 
that can be used as an intake-valve spring or vice versa. But if 
there is any difference to be detected in the spring tensions, the 
weak springs are to be placed upon the intake and the strong ones 
upon the exhaust. The valve-spring tension increases as the valve 
lifts, therefore motor data as to spring tensions is always given for 
the compressed length of the spring when the valve is seated. Two 
valve springs per valve are not unusual, and afford a construction 
giving a greater factor of safety; for if one spring should break the 
other would still seat the valve at low speeds and keep the valve 
from falling into the cylinder. It may also be said that with the use 
of two valve springs a heavy, high-speed valve can be returned to 
its seat with less wear upon the face than if one extra-strong spring 
were used. 

No valve springs should be placed without being first tested. This 
testing is quickly performed by the use of a common scale. By com- 


GASOLINE ENGINES. 


41 


pressing the spring upon the scale platform to its proper compres¬ 
sion length upon the motor, with valve seated, the spring may be 
“ weighed,” the number of pounds being the compression pressure 
of the spring. 

Even if the data for the springs of a motor are not available, 
weak springs are readily detected by weighing them all and picking 
out the ones below the average. If scales are not obtainable, it is 
possible to take two springs and place them end to end between 
the jaws of a vise, noting which one compresses the more. Com¬ 
pare the stronger of the two with the rest, etc., when finally a de¬ 
termination of the weak springs can be made. 

On some motors the valve-operating mechanism is so designed that 
the intake valve is opened by spring pressure and closed by combina¬ 
tion spring and cam action. In these cases the heavy springs for 
valve opening must be weighed in very accurately. 

CASE HARDENING VALVE-STEM ENDS. 

It is frequently necessary to “square up” valve-stem ends when 
they have been slightly hollowed out by the action of a contact screw 
head against them. The squaring-up process, usually grinding a bit 
off, removes the case hardening. The rehardening is commonly done 
with cyanide of potassium. One of the best ways to heat the stem 
so as not to warp it is by the use of a lead bath. The lead is heated 
past its melting point on up to a red heat, then just the end of the 
stem held in it until it is red hot. The depth of the dip should be 
about one-eighth inch. The stem is then dipped into powdered cya¬ 
nide and immediately quenched in cold water. Do not heat the stem 
any farther up than necessary. The end is all that needs hardening, 
and this process gives a very thin, hardened layer to the end. If it is 
desirable to get the valve stem hardened deeper, it may be held until 
red hot at the end in a molten bath of cyanide for about 10 minutes, 
then quenched in cold water, being careful to dip the stem in the 
water vertically. 

cams. 

The figure below shows the common type of cams that are ground 
integral with the cam shaft. The exact shape of the cam is very 
important, and it is impossible to detect wear of even a serious 
nature by the eye in noting any change from the original shape. 
Usually wear of the cams is most noticeable just at the valve-opening 
period or a point practically where the cam engages the cam roller. 
By a light, quick movement of the fingers over the pick-up point 


42 


GASOLINE ENGINES. 


referred to above, any little hollowing out of the metal can be felt 
when it can not be seen. The only practical method the mechanic 
has of detecting cam wear is by the checking up of valve timing. 
It is therefore highly important that when possible the valve timing 



should be recorded for a motor prior to dismantling. Then by refer¬ 
ence to the shorting of any valve-opening period an idea of cam-shaft 
condition may be obtained. 


o 














L[BRA ry of congress 


0 033 266 783.A 


